I decided that I wanted to use a 5-speed transmission so as to have an overdrive. The T-5 would be marginal in its ability to handle the torque, so I found a Tremec five speed (Model 3550 2600682) on CraigsList. These trannys, used in the Mustang R models (circa '94) are rated at 375 foot pounds of torque, plenty to handle a modified 289. The main downside to these is that they are heavy. All by itself, it is 105 Lbs. The 240Z transmission and bell housing is only 74 Lbs. Fortunately, it is right in the middle of the car so it doesn't add to any weight imbalance or to the polar moment of inertia.
It has a back-up light switch that will work with the Datsun wiring (when slightly extended and with a different connector), and also has a switch towards the rear that indicates neutral. If one was so inclined, he could wire this into the starter circuit so that it couldn't be started in gear, but that would require a relay to reverse the polarity of the switch – it is normally closed until you go into neutral, then it opens. I believe it was originally used to kick out the cruise control if you went to neutral.
These transmissions have a 10-spline 1 1/16" input shaft with a standard Ford 28-spline output shaft.
Purchase of the transmission was interesting. I answered an ad on CraigsList for a T5, and then got the address via email. I assumed it was a private party, but when I got there, I discovered that it was a used car lot with several Mustangs inside being disassembled by Russians. I was shown the Tremec, and was happy to see that it included a Pro-5.0 tower shifter, and he would throw in a bell-housing and new clutch cable with the deal. So I got a receipt and made the nasty drive home. When I got home, the phone was ringing, and it was the seller. He said that they had made a big mistake and he had given me a shot core transmission that was in shambles. I needed to make the long drive back and exchange it for a good one. Before I jumped back into the truck, I decided to investigate further. The shafts moved smoothly in all the gears. I took the cover off, and found that it looked like brand new inside. Then I noticed that it had a nice cardboard tube over the input, a rubber plug over the speedometer drive, and new gasket sealer. I guess he gave me a rebuilt tranny, and wanted to exchange it for a lesser one. Well, had he not lied about it, I might have gone back. I'm looking over my shoulder to watch for Russian mafia guys though!*
I spent about half an hour cleaning up the incredibly filthy bell housing he had given me, and then discovered that it was for a T5 rather then Tremec – the two are different in bolt pattern for the bell housings, and also a different distance from the front of the transmission to the pilot bearing. Evidently the Tremec is made to match the early 289 four-speeds in function.
I ordered a new bell housing from Modern Drivelines in Nampa Idaho. Very friendly, knowledgeable folks there. They deal in transmission and driveline conversions.
I purchased a transmission mount at the local parts store.
*Note that I don't have any inherent dislike for Russians. I've worked with Russians professionally and visited Russia a couple of times - and they were delightful people! Unfortunately, a disproportionate amount of current Russian immigrants seem to be involved with some of the darker shades of the automotive industry.